1. Field of the Invention
The present invention relates to a code division multiple access (CDMA) system, and more particularly to a rake receiving apparatus used in communication systems using a direct sequence CDMA (DS/CDMA) system.
2. Description of the Prior Art
Generally, such a rake receiving apparatus is known to provide an improvement in performance as compared to cases wherein a single demodulator is used. The rake receiving apparatus operates to detect the phases of pseudo noise (PN) codes of signals respectively corresponding to multipaths by use of a searcher. The signals from the multipaths are demodulated in corresponding independent demodulators by use of a plurality of demodulating fingers as receivers, corresponding in number to the multipaths, respectively. The results obtained from the receivers after the demodulation are coupled together in a combiner to extract desired timing information and demodulated data. Thus, the rake receiving apparatus can obtain an improvement in performance, as compared to cases using a single demodulator.
Referring to FIG. 1, a conventional arrangement of such a rake receiving apparatus is illustrated. As shown in FIG. 1, the rake receiving apparatus includes a single searcher, a plurality of demodulating fingers and a post-processing combiner (adapted to extract demodulation data, timing information and frequency information). In this rake receiving apparatus, the searcher first searches for the phases of PN codes corresponding to nultipaths. Based on the result of the operation of the searcher, the demodulating fingers carry out their demodulation operations. The demodulating fingers operate individually to demodulate signals associated therewith. The combiner combines the results of the modulation operations respectively carried out in the demodulating fingers, thereby extracting required data and timing information. Since each finger must execute the whole procedure revired for a reverse sequence process, such a rake receiving apparatus using N receivers achieves an improvement in performance as compared to the demodulating method using a single receiver. In this case, however, there is a problem in that the complexity of hardware increases N times or above.
Meanwhile, systems with as fixed number of fingers involve a problem in that they can not demodulate signals from multipaths which are more than the given number of fingers.